Hot water tanks with a small capacity, such as domestic hot water heaters, have generally been provided with a passive anti-corrosion layer, mostly of enamel or glass, and a sacrificial or donor anode of magnesium, zinc or aluminum. The sacrificial anode is usually screwed through a socket in the top of the tank at the time the tank is manufactured. Although coatings are a major form of corrosion control, no coating is perfect. All coatings have at least some porosity to water and chloride ions, in many cases through pinholes and other mechanical defects in the coating.
It is a well established fact that the life of a domestic hot water heater can usually be calculated based on the life of the sacrificial anode used to protect it from corrosion. The warranties associated with the water heater industry are based on some average theoretical life of the sacrificial anode or anodes installed by the water heater manufacturer. The water heaters with long warranties simply have more sacrificial anodes in them. When the anodes are expended (sacrificed), the life of the water heater from that point on is very tenuous. In some geographical areas, the water is so corrosive that the sacrificial anodes will not last six months, much less the life of the warranty on the tank. The corrosivity of the water to sacrificial anodes generally depends on electrical resistivity, pH, copper ions, oxygen concentration, water hardness, and other factors such as sulfate reducing bacteria. Sacrificial anodes are also generally ineffective for protecting the parts of the water tank which are remote from the anode. Thus, the size and shape of each tank is an important factor in determining the number of anodes and the size of anodes necessary to adequately protect the entire tank.
A commonly used corrosion prevention technique is cathodic protection, involving the application of a small electric current from an external source to the corrodible structure. The current is supplied through the cathodic anode and eventually consumes it. The anode is the positive terminal in the cathodic protection circuit, and the structure is the negative terminal. Many types of impressed current anodes are available. U.S. Pat. No. 3,379,630 discloses the use of coiled aluminum anodes, suspended vertically in an aluminum alloy tank containing alkali nitrate solution, in order to provide cathodic protection. However, the aluminum anodes are not considered to be suitable for use in hot water tanks. Silicon-iron and graphite have been in widespread use for the fabrication of cathodic anodes. However, these materials are brittle and have consumption rates on the order of 200 to 450 grams per ampere-year in contrast to consumption rates of 40 grams per ampere-year for cast magnetite and 0.01 gram per ampere-year for platinum coated titanium.
U.S. Pat. No. 4,039,417 discloses the use of a platinum coated titanium wire or a ferrite sintered body. Platinum has the lowest anodic dissolution rate of any material, but the dissolution rate increases substantially at elevated temperatures such as are encountered in hot water tanks. Platinum is also disadvantageous because of its very high cost and the susceptibility of a thin coating of platinum to erosion or abrasion. Whenever a substrate of a valve metal such as titanium, tantalum or niobium is exposed, a stable, passivating oxide film forms which prevents the flow of the current from the anode region where the platinum coating is interrupted. The ferrite ceramic materials have iron oxide as the principal component, and have good corrosion resistance, with a dissolution rate of 1 to 10 grams per ampere-year. However, the ferrite ceramic anodes are inherently brittle and are not acceptable where the anode might be mechanically damaged. The ferrite sintered body electrodes are usually cast as hollow cylinders with one end closed and the entire inner surface layered with electrodeposited copper to distribute the current evenly over the entire anode. They can also be produced as plates or rods. Generally, cast anodes produced by sintering are very brittle and are restricted in shape and size. This limits the feasibility of using such anodes for domestic water heaters, particularly with regard to the replacement market.
U.S. Pat. Nos. 4,407,711 and 4,434,039 disclose the use of an elongated anode having an outer layer of platinum, iridium, ruthenium or their alloys coated on a strand of titanium, columbium or tantalum disposed on an electrically insulative support One embodiment is an insulative polypropylene rod having an axially extending channel which receives the anode strands, while another embodiment has the anode strands wrapped helically about a tube. The noble metal is expensive, and the anode strands are readily subject to mechanical damage.
Conducting ceramic coatings are a relatively new concept in the field of impressed current anodes. The conducting ceramic anode coating must provide an effective barrier to oxygen ions, so that the substrate metal does not become oxidized In addition, the ceramic coating must have a relatively high electron conductivity on active surface area for oxidation to occur, be mechanically strong and have good adherence to the substrate. U.S. Pat. No. 3,850,701 describes the use of a magnetite coating which was chemically processed over a titanium or tantalum substrate. However, the resulting coating had insufficient adhesion to the substrate.
U.S. Pat. No. 4,445,989 discloses the use of a second type of ceramic anode. It employs a cathodic anode comprising an electrically conducting ceramic coating on a valve metal substrate, where the consumption rate of the ceramic coating is on the order of 1 gram per ampere-year. The ceramic coating is approximately 10 to 20 mils thick and is produced by plasma spraying. The ceramic coating is either ferrite or chromite, while the preferred valve metal substrates are titanium and niobium.
U.S. Pat. Nos. 3,846,273 and 3,948,751 describe titanium or niobium metal substrates coated with niobium-doped titanium oxide by conventional techniques. U.S. Pat. Nos. 4,112,140 and 4,214,971 disclose a ruthenium oxide-titanium oxide coating on a valve metal substrate produced by conventional techniques.
Later it was discovered that composite anodes having excellent characteristics of low resistivity, very low dissolution rates, long life, durability, and corrosion resistance can be produced by reactive ion plating a thin layer of mixed metal oxides on a self-passivating electrically conductive valve metal base. The mixed metal oxides are composed of transition metal oxides and/or noble metal oxides of the platinum group. The valve metal is generally titanium or niobium. Three preferred embodiments are a niobium-doped titanium oxide coating or a ruthenium oxide/titanium oxide mixture coating or an iridium oxide/titanium oxide mixture coating on a niobium or titanium valve metal substrate. The thickness of the coating is at least one micron and can be 50 microns or greater. The coating is preferably achieved by reactive ion plating in concert with predeposition sputter cleaning of the substrate surface.
U.S. Pat. No. 4,231,852 discloses the cathodic protection of a hot water tank wherein a reference electrode, positioned close to the tank wall, is used to control the current flowing through the cathodic anode. The reference electrode is positioned coaxially with the anode and both are supported by a common holder so that the combination can be inserted through a single opening in the tank wall.